Stem cells are the pivotal cells in development and malignancy. In normal mammalian development, they undergo regulated cycling and differentiation; in neoplasia, proliferation occurs at the expense of differentiation. The objectives of the program are to increase our understanding of these cells and to learn how genes influence their activities. Among several systems that will be studied in mice, the hematopoietic system has the advantage that the stem cells can be replaced prenatally and will then clonally give rise to all blood lineages. The proliferative and possible tumorigenic roles of cellular proto-oncogenes such as c-myc will be investigated by introducing them into the stem cells in culture, in new retroviral vectors, before cell inoculation into fetuses. The fusion gene constructs are designed to include tissue-specific internal controlling (enhancer and promoter) sequences, rather than those of the viral long terminal repeat; and a marker gene enabling preselection of positive cells in culture. The effects of different transcriptional controls on tissue specificity of oncogene expression, and their relation to formation of tumors, will also be studied in a wider context by producing transgenic mice from fertilized eggs injected with DNA. A bacterial enzyme gene will be inserted in these constructs to visualize histochemically the specific cell populations in which the oncogene is expressed. Other experiments with hematopoietic stem cells are directed at the question whether control of cell proliferation is exerted by genes, possibly in the H-2 region, that are expressed in cells of the bone marrow environment, apart from effects of genes expressed in the stem cells themselves. The pigmentary system provides another unique advantage for the study of stem cells: the melanoblasts form coherent clones whose color, geometry, and viability are influenced by various genes and are suggestive of heterogeneity originating in the stem cells. To learn the molecular genetic basis for this heterogeneity, and to clarify the stem cell origins of pigment cells and of their conversion to melanomas, new gene and antibody probes will be produced for in situ detection of the stem cells and of their malignant and metastatic derivatives, by cloning the mouse tyrosinase gene and several of its mutant alleles. The stem cell origins of other tumors that are rich in alkaline phosphatase will also be examined with the alkaline phosphatase gene as a probe for revealing early energy changes in preneoplastic tissues.